The invention relates generally to a combined cycle operation and more particularly, to a method and system for controlling a drum water level during start up operation of a heat recovery steam generation (HRSG) system for a combined cycle power plant.
Generally, the combined cycle power plant includes gas turbines, steam turbines and the HRSG systems. The HRSG systems are basically used for recovering heat in exhaust gases from the gas turbine and then using the recovered heat to produce and supply steam to the steam turbine. In general, the combined cycle power plant includes various kinds of water reservoirs such as boiler drum, dearator reservoir and condensor reservoir. Normally, the boiler drum provides a stable water supply to the HRSG system and accumulates heat energy for compensating for changes in the amount of steam generated or consumed in the power plant. The boiler drum water level control is critical for both plant protection and equipment safety and applies equally to high and low levels of water within the boiler drum. Additionally, a water vapor separator is especially provided inside the boiler drum so that wet steam may not be introduced into the steam turbine. Maintaining boiler drum water level within limits is important as exceedance will often lead to trips or damage the equipment used in the HRSG system.
Moreover, in a starting operation of the combined cycle power plant, the water level in the boiler drum rises abruptly. This start up drum level swell is because, when the water in the HRSG system is heated with the high temperature gas turbine exhaust gases, the temperature of the water in the HRSG system rises rapidly and evaporation starts suddenly. In this state, vapor phase and liquid phase coexist in the steam generator. Since the average specific volume (m3/kg) of the water is very small compared to steam, this causes an abrupt water level rise in the boiler drum. This water level rise may exceed the highest water level allowed and thereby create a safety issue. Accordingly, when this water level rise happens, it is necessary to blow out the excessive water from the drum. However, the water level rise is so rapid that delicate controls are necessary to blow out the excessive water, otherwise safety problems are encountered especially in cases where the combined cycle power plant is periodically required to start up and shut down operation every day. Also, heat losses associated with blowing out the excessive water affects the efficiency of the combined cycle power plant.
Thus, to prevent the swelling phenomenon of the boiler drum the water level of the drum is lowered in advance of a plant start up operation. Generally, the water level of the boiler drum is controlled to approximately maintain a constant water level by adding water into or blowing water out of the boiler drum. However, the control of boiler drum water level is a challenging problem due to complicated dynamics of two-phase flows, the presence of waves, unknown heat and pressure disturbances and load demands.
Therefore, it is necessary to efficiently control the water level in a boiler drum during start up operation of the HRSG system for a combined cycle power plant.